1. Field of the Invention
The present invention relates to a recording apparatus and a method for recording, and more specifically, it relates to a technique to correct a recording medium conveyance amount error used in an ink jet recording apparatus.
2. Description of the Related Art
In an ink jet printer, a high-precision roller that is a metal shaft coated with grindstone has been used as a main conveying roller, and a DC motor has been controlled using a position detecting unit (a code wheel and an encoder sensor) provided on the axis of the roller. Thus, in an ink jet printer, a recording medium (paper) can be conveyed with a high degree of accuracy, and a high-quality image can be recorded. However, improving the paper conveyance accuracy by improving the accuracy of processing of the conveying roller is approaching a limit. To solve this problem, recently, for example, roller eccentricity correction has been performed.
The eccentricity correction will be briefly described. When a conveying roller has a circular cross-sectional shape and the central axis thereof corresponds to the rotation axis, and when the roller rotation angle for paper conveyance is uniform, the length in the circumferential direction (the length of an arc) when the roller is rotated is constant. Therefore, the conveyance amount of a recording medium conveyed in contact with the roller is constant. However, when a conveying roller has an elliptic cross-sectional shape, the conveyance amount per given rotation angle of the roller varies depending on the rotational position (rotation phase) of the roller. That is to say, there are regions where the conveyance amount is larger than a predetermined amount and regions where the conveyance amount is smaller than the predetermined amount, depending on the rotation phase of the roller, and the conveyance amount error fluctuates. In the eccentricity correction, the conveyance amount correction value of each rotation phase of the roller is obtained, and the conveyance amount error fluctuating depending on the rotation phase is corrected. In the following description, a conveyance amount when a roller is rotated by a given angle will be also referred to as unit conveyance amount.
A discharge roller disposed downstream of the conveying roller conveys paper after ink is shot onto the paper. For this purpose, the discharge roller is provided with a star-shaped driven roller called spur. The discharge roller is formed of an elastic material (rubber) so as not to damage the spikes of the spur. Even if the eccentricity correction is performed, the same paper conveyance accuracy as the conveying roller cannot be maintained.
What is especially important for paper conveyance accuracy is the conveyance amount at the time of transfer from the conveying roller to the discharge roller. That is to say, the paper conveyance accuracy at the time of switching from a state where paper is conveyed by both the conveying roller and the discharge roller to a state where paper is conveyed only by the discharge roller, is important. It is known that the accuracy of the conveying operation at this time is generally lower than the accuracy of the first conveying operation due to various factors such as deflection of roller shaft and instability when paper leaves the conveying roller in addition to the factor of roller precision error. Japanese Patent Laid-Open No. 2005-7817 discloses a technique to reduce the reduction in conveyance accuracy at the time of transfer from the conveying roller to the discharge roller. In this technique, the conveyance amount correction value at the time of transfer is obtained using a test pattern, and the conveyance amount at the time of transfer is corrected using the obtained correction value.
As described above, due to the roller eccentricity, the conveyance amount error fluctuates depending on the roller rotation phase. This phenomenon also occurs in the conveying operation at the time of transfer. Depending on the rotation phase of the conveying roller and the rotation phase of the discharge roller at the time of transfer, the error in conveyance amount at the time of transfer also fluctuates.
However, in the method of Japanese Patent Laid-Open No. 2005-7817, the correction value of conveyance amount at the time of transfer is a fixed value, and in the conveying operation at the time of transfer, the correction control of conveyance amount is always performed using the fixed correction value. Therefore, in the conveying operation at the time of transfer, the roller rotation phase varies by conveying operation. If the error in conveyance amount at the time of transfer fluctuates depending on the rotation phase, the error cannot be accurately corrected.